It seems YT started another attempt at blocking alternative clients. They changed something in their API and both SmartTube and Tubular (NewPipe fork) are completely broken. Apparently it started happening this past week, but we personally just felt it today.
Edit: SmartTube already has an update but still not working for 4K videos it seems. Tubular still not working but it might be due to the upstream (NewPipe) is still working on a fix.
Since yesterday for me on Tubular. No video seems to load but live streams still work fine.
SmartTube was working before I went to work and now it isn’t. I’m not too worried or upset. Yet.